Separation and detection of trace substances in gaseous samples containing moisture by diluting with dry air

ABSTRACT

Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions and for reducing the deleterious effects of moisture upon the measurements. Positive or negative ions of the trace gas are formed by ion-molecule reactions between molecules of the trace gas and primary ions from another gas. Ions are classified in accordance with their velocity in an electric drift field. The effects of moisture in the sample are significantly reduced by diluting the sample with dry air, which is recirculated after treatment.

[ June 6,1972

United States Patent Cohen et al.

Primary Examiner-Archie R. Borchelt TRACE SUBSTANCES IN GASEOUS Assistant Examiner-C. E. Church SAMPLES CONTAINING MOISTURE BY Am""ey Raphael Semmes DILUTING WITH DRY AIR [72] Inventors: Martin J. Cohen, West Palm Beach; Roger I54] SEPARATION AND DETECTION OF ABSTRACT F. wemhmd Lake worth; David L can Ap aratus and methods for sorting and detecting trace gases which undergo ion-molecule reactions and for reducing the roll, Lantana, all of Fla.

deleterious effects of moisture upon the measuremen ts. Posi- Franklin Gno Corporation, West Palm tive or negative ions of the trace gas are formed by ion- Beach, Fla. molecule reactions between molecules of the trace gas and [73] Assignee:

lons are classified in acprimary ions from another gas.

[22] Filed: Nov. 26, 1968 electric drift field. The efre significantly reduced by cordance with their velocity in an fects of moisture in the sample a [2]] Appl. No..:

diluting the sample with dry air, which is recirculated after treatment.

[52] US. [51] Int. [58] Field of m u b F g .m w m 2 .m M C, h m Mm P S 2. PM .ms m M .U M b.

3,355,587 12/1967 Jenckel...7.........................25O/41.9G

PATENTEBJUN 6 I972 FIG! SYNC

PULSE INVENTORS MARTIN J- COHEN ROGER E WERNLUND DAVID I. CARROLL ATTORNEY SEPARATION AND DETECTION OF TRACE SUBSTANCES IN GASEOUS SAMPLES CONTAINING MOISTURE BY DILUTING WITH DRY AIR BACKGROUND OF THE INVENTION .No. 777,964 filed Oct. 23, 1968 and entitled Apparatus and Methods for Separating, Concentrating, Detecting, and Measuring Trace Gases," discloses Plasma Chromatography systems involving the formation of primary reactant ions and the reaction of such primary ions with molecules of trace substances to form secondary or product ions, which may be concentrated, separated, detected, and measured by virtue of the difference of velocity or mobility of the ions in an electric field. The primary ions may be produced by subjecting the molecules of a suitable host gas, suchas air, to ionizing radiation, such as beta rays from a tritium source, corona from a multi-point or wire array, electrons produced by photoemission from a cathode, etc. The'primary ions are then subjected to an electric drift field, causing them to migrate in a predetermined direction through a reaction space into which the sample or trace gas is introduced. The resultant collisions between primary ions and the trace gas molecules produce secondary ions of the trace gas in much greater numbers than can be produced by mere electron attachment, for example, to the trace gas molecules. The secondary ions are also subjected to electric drift field and may be sorted in accordance with their velocity or mobility. The gas pressure in the Plasma Chromatograph drift cell is maintained high enough (preferably atmospheric) to ensure that the length of the mean free path of the ions is very much smaller than the dimensions of the cell. The specific systems of the said copending application employ ion shutter grids or gates for segregating the ion species in accordance with their drift time.

Laboratory instruments operating upon the foregoing principle have been found to operate very effectively in detecting ions of complex molecules, such as certain gaseous body effluents, with sensitivity of the order of one part in in dry air. When such instruments are utilized with moist air, as would be encountered in the field, the sensitivity is somewhat less. Moreover, it has been discovered that the moisture acts as an interferant which tends to reduce the resolution of the instrument and thus its ability to separate and detect different ion species.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal object of the invention to provide improved apparatus and methods for separating and detecting molecular quantities of trace substances in the presence of moisture and to provide apparatus and methods which significantly reduce the interference effects of moisture upon the measurements.

Briefly stated, preferred embodiments of the apparatus and method of the invention are concerned with Plasma Chromatography systems, which involve the formation of positive or negative ions by reactions between the molecules of trace substances and primary ions. The secondary ions may then be separated and detected. Separation is accomplished in a drift cell by utilizing the difference in velocity or drift time of ions of different mass in an electric field. In order to reduce the obscuring effects of moisture in the sample gas upon the output of the drift cell, the sample gas is diluted with clean dry gas before being fed to the cell. The gaseous exhaust from the cell is recirculated, after treatment, for mixture with a new gaseous sample.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described in conjunction with the accompanying drawings, which illustrate a preferred and exemplary embodiment, and wherein:

FIG. 1 is a perspective view, partly in section, and somewhat diagrammatic, illustrating a trace gas detector system of the invention; and

FIG. 2 is a diagrammatic view illustrating a Plasma Chromatography cell and operating circuit utilized in the invention.

DETAILED DESCRIPTION OF THE INVENTION Before the invention is described in detail, reference will be made to FIG. 2 for an understanding of the basic Plasma Chromatography cell which is preferably employed in the invention and which is described in detail in theaforesaid copending application. The cell 10 comprises a gas-tight envelope, indicated by phantom lines 12, enclosing a series of electrodes, which may be of plane parallel geometry. Principal electrodes 14 and 16 may be arranged adjacent to opposite ends of the envelope, which may be a circular cylinder. When the apparatus is used to detect negative ions, as will be assumed for example, electrode 14 will be a cathode and electrode 16 an anode. When the apparatus is used to detect positive ions, the polarities will be reversed. As described in the said copending application, the Plasma Chromatography cell preferably includes a pair of shutter grids or ion gates 18 and 20, each of which comprises two sets of interdigitated parallel wires, alternate wires of each grid being connected together to form the two sets. Cathode 14 or the region of the envelope near this electrode is provided with a source of ionizing electric charge, which may be of the type mentioned previously. Anode 16 may be a collector plate connected to an output device, such as an electrometer 22, which may be Cary Instruments Model 401 vibrating reed) type with current sensitivity of 10 amperes at a time constant of 300 milliseconds.

An electric drift field is provided between the principal electrodes 14 and 16. In the form shown the source of the drift field is a battery 24 having its positive terminal connected to ground and its negative terminal connected to .cathode l4. Anode 16 is connected to ground through the input circuit of the electrometer 22. A resistor chain voltage divider 26 is connected between the negative terminal of battery 24 and ground. Taps on the voltage divider are connected to a series of guard rings 28 spaced along the length of the envelope 12, which maintain the uniformity of the drift field.

Adjacent elements of each shutter grid 18 and 20 are normally maintained at equal and opposite potentials relative to a grid average potential established by the voltage divider 26.

Under these conditions the shutter or gate is closed to the passage of electrically charged particles. The potential sources which provide the equal and opposite potentials just referred to may be considered to be part of grid drive circuits within blocks 30 and 32 entitled Sync Pulser and Delayed Pulse. The components of these blocks are effective to drive the adjacent elements of each shutter grid to the same potential, the grid average potential, at predetermined instants, alternate grid wires being shown connected to the resistor chain by resistors 34 and 36 to establish the grid average potentials.

As described in the said copending application, a suitable host gas, such as air, carrying an appropriate gaseous trace substance, such as certain body effluents, for example, flows through the envelope by means of a gas inlet 38 and a gas outlet 40. Any suitable source of flow pressure, such as a fan or pump, may be employed to move the gaseous sample. In the region between the cathode 14 and the first grid 18 primary ions of the carrier gas, or one or more of the main constituents thereof, such as oxygen, are formed under the influence of the ionizing means in this region. For example, negative oxygen ions may be formed at cathode 14, as by direct attachment of electrons to the oxygen molecules, the sample being subjected to beta rays produced by a tritium source on the cathode.

The air in the drift cell may be moved through the ion reaction region at a velocity of 5.0 centimeters per second, for example. If the drift space diameter is 5 centimeters, the entire reaction region is changed once every 100 milliseconds. The actual reaction volume is much smaller, since it is determined by the tritium source size. If this is about I centimeter, the sample is effectively jchangingof the order of 50 times per second.

The primary ions drift toward the anode, and in the reaction space between the cathode and the first shutter grid 18, the primary ions encounter other molecules (at the rate of per second in air at atmospheric pressure). A majority of these collisions will be with oxygen, nitrogen, or other non-reactive molecules. A small fraction of the collisions will be with the trace molecules .of interest. In these cases, the primary ions will interact with the trace molecules to form secondary ions. The secondary ions will have, in general, an appreciable difference in mobility from the primary ions.

Under these conditions the ion flux atthe first shutter grid 18 will consist of both the primary ions and possibly'several species of secondary ions; A sample of this mixed ion population is periodically admitted to the drift region between the first and second shutter grids when the first shutter grid 18 is opened by momentarily driving all of the grid wires to the grid average potential. The second shutter grid 20 is opened for a predetermined interval at a predetermined time after the opening of the first grid. The ions that pass through the second grid'drift toward and are collected by the anode l6, and the resultant output. current is integrated over several cycles to give a measurable current. By scanning the time of opening of the second grid relative to the .first, a drift time spectrum of the ion population can be obtained inthe output and recorded (current vs drift time). This permits the various ion species to be separated and identified. Additional ion-molecule interactions canoccur in the drift region between the grids. Because such interactions can occur at any point in this drift space, the resulting ions will not be spacially coherent and will not give rise to unique peaks in the output current. These interactions will, however, give rise to a tail on the primary peak upon which the secondary ion peaks ride.

Utilizing the Plasma Chromatography technique just described, many substances can be measured with great sensitivity, either as negative or positive ions. Measurements can be made in approximately one-tenth of a second on a small sample'of gas'and without preconcentration of the sample. Sensitivity of the order of one part per billion is obtainable without fragmentation of the trace molecules.

In making measurements of trace substances carried in air it has been discovered that water vapor in the ambient atmosphereis by far the largest source of interference. The water vapor appears to form complex secondary ion clusters with oxygen of the form:

2 )n where n ranges from 0 to 6 or more. Similar interactions apparently occur to a lesser extent with complex secondary ions. It has been found that moisture can generate a succession of output current peaks with O ranging from 2.31 to 1.68 cm*/ Vs in reduced mobility. Since these peaks are closely spaced, they can obscure other peaks in the same range and can thus obscure peaks produced by the trace gas ions of interest. In many cases the instrument response for specific materials is entirely obscured at a water partial pressure of several torr. It has been discovered that specificity and sensitivity can be maintained by reducing the water vapor pressure in the drift cell to the 'order' of two torr ('10 percent relative humidity at 23 C. )or less. This is accomplished in accordance with a dilution technique of the invention which will now be described.

Referring now to FIG. 1, the drift cell 10 is shown surrounded by a thermal insulating blanket 42 within an outer protective shell 44. The output connection to the electrometer is shown at 46. The grid drive circuitry and other electronics may be housed within a casing 48. In accordance with the invention the gas at the outlet40 of the drift cell is returned to the inlet 38, after treatment, by a recirculation loop generally designated by reference numeral 50. A pump 52, which may be supported upon the housing 44, receives air from the outlet 40 by means of conduit 54 and the associated couplings and forces the air through a dehydration tube 56 in the directionof arrow A at a suitable flow rate, such as 1 cubic foot per minute. The dehydration tube may containa bed of LMS-5A molecular sieve, which removes all contaminants from the air, including the candidate or trace material and moisture. A conventional pressure relief valve (not shown) may vent excess air to the atmosphere by means of outlet 58. A control valve (not shown) following the dehydration tube 56 may be employed to control the flow rateto a sample mixer 60. The sample mixer is essentially a venturi which draws in the sample such as ambient air and trace material and mixes it in controlled proportion with the dry air provided by the recirculation loop. The sample inlet pipe 62 may be internally screened to prevent ingestion of particulate material, and the inlet 38 to the envelope of the drift cell may be provided with internal screens or grids to reduce turbulence. A dust filter may be provided in section 64 of the recirculation loop.

The diluted sample is passed through the reaction region of the drift cell, as described previously, where the candidate or trace material in the host gas forms ions which are removed from the air flow by the electric drift field. The drift field is at right angles to the gas flow, and the ions are rapidly removed from the effects of the flow. These ions may be analyzed by means of the two shutter grids described above.

The sample in the reaction region is removed, cleaned, and dried and recirculated to the sample mixer by the pump 52. In this way most of the gas mixed with the sample is recirculated, which lowers the amount of molecular sieve required. The sample mixer 60 and the envelope of the drift tube are preferably heated by means of a heating mantle, such as heater wires 66 and 68 which surround the respective elements. Thermal insulation 70 may also surround the sample mixer as shown. The heating of these elements prevents accumulation of material on the walls which would tend to lower the speed of response." 7 v 2 By virtue of the invention the obscuring effects of moisture in the sample are significantly reduced. Although the dilution of the sample reduces the sensitivity somewhat, the improvement in resolution justifies the loss of sensitivity. Moreover, the employment of the ion-molecule reaction principle to produce the trace ions improves the sensitivity sufficiently, in comparison with prior techniques,to render acceptable the loss of sensitivity due to dilution.

While a preferred embodiment of the invention has been shown and described, it will be apparent to those skilledin the art that changes can be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.

The invention claimed is:

I. In a method of detecting a trace material in a gaseous sample containing substantial quantities of moisture and in which the detection is a function of the mobility of ions from the gaseous sample produced by ion-molecule reactions at substantially atmospheric pressure and is subject to interference by said moisture, the improvement which comprises diluting the gaseous sample by adding dry gas thereto substantially devoid of trace material prior to detection of ions from the gaseous sample and until the partial pressure of the water vapor is of the order of 2 Torr or less 2. A method of detecting a substance in a gaseous sample- 3. A method in accordance with claim 2, in which the detecting is accomplished by selectively gating a group of said ions to a first region at a predetermined time, and at a predetennined time thereafter selectively gating a portion of said group from said first region to means for detecting said portion.

4. A method in accordance with claim 2, in which said gaseous sample comprises air and in which said dry gas is air.

5. A method in accordance with claim 2, and in which the sample is diluted until the relative humidity of the sample is percent or less.

6. A method of reducing the effects of moisture upon ionvelocity-dependent measurements in a drift cell, which comprises mixing a sample of moisture-containing gas and tracesubstance-containing gas to be analyzed in said drift cell with dry gas substantially free of said trace substance, feeding said mixture to said cell, forming ions from said mixture for analysis in said cell, exhausting the remainder of said mixture from said cell, removing any trace substance in the exhaust from said cell, dehydrating the exhaust from said cell, and utilizing the resultant dehydrated exhaust free of said trace substance to mix with a new sample to be analyzed in said cell.

7. Apparatus for detecting the presence of a substance in a gaseous sample in the presence ofsubstantial quantities of water vapor, which comprises an envelope having a gas inlet to which said sample is fed, means for forming ions from said sample in said envelope by ion-molecule reactions, means for applying a drift field to the ions in said envelope, means for producing an electrical output from said envelope in accordance with the velocity of predetermined ions in said drift field, means for mixing said gaseous sample with dry gas substantially free of said substance before said sample is fed to said inlet and until the partial pressure of said water vapor is of the order of 2 Torr or less, and means for maintaining substantially atmospheric pressure in said envelope.

8. Apparatus in accordance with claim 7, further comprising means for heating said envelope and said mixing means. v

9. Apparatus in accordance with claim 8, wherein said envelope and said mixing means are surrounded by thermal insulation.

10. Apparatus in accordance with claim 7, wherein said mixing means comprises a venturi.

l 1. Apparatus for detecting the presence of a substance in a gaseous sample, which comprises an envelope having a gas inlet to which said sample is fed, means for forming ions from said sample in said envelope, means for applying a drift field to the ions in said envelope, means for producing an electrical output from said envelope in accordance with the velocity of predetermined ions in said drift field, and means for mixing said gaseous sample with dry gas substantially free of said substance before said sample is fed into said inlet, said envelope having a gas outlet connected to said mixing means in a recirculation loop containing means for removing said substance.

12. Apparatus in accordance with claim 11, wherein the last-mentioned means includes a molecular sieve.

13. Apparatus in accordance with claim 11, wherein said recirculation loop contains dehydrating means.

14. Apparatus in accordance with claim 11, wherein said recirculation loop includes means for forcing gas therethrough.

15. Apparatus in accordance with claim 11, wherein said recirculation loop has means for venting excess gas to the atmosphere. 

2. A method of detecting a substance in a gaseous sample containing a substantial quantity of moisture interferant, which comprises diluting the sample by adding dry gas thereto substantially devoid of trace material until the partial pressure of the moisture interferant is of the order of 2 Torr or less, forming ions from the sample including ions of the said substance, by ion-molecule reactions, applying a drift field to said ions, and detecting ions having a predetermined velocity, in said field, said forming and field-applying steps being performed at substantially atmospheric pressure.
 3. A method in accordance with claim 2, in which the detecting is accomplished by selectively gating a group of said ions to a first region at a predetermined time, and at a predetermined time thereafter selectively gating a portion of said group from said first region to means for detecting said portion.
 4. A method in accordance with claim 2, in which said gaseous sample comprises air and in which said dry gas is air.
 5. A method in accordance with claim 2, and in which the sample is diluted until the relative humidity of the sample is 10 percent or less.
 6. A method of reducing the effects of moisture upon ion-velocity-dependent measurements in a drift cell, which comprises mixing a sample of moisture-containing gas and trace-substance-containing gas to be analyzed in said drift cell with dry gas substantially free of said trace substance, feeding said mixture to said cell, forming ions from said mixture for analysis in said cell, exhausting the remainder of said mixture from said cell, removing any trace substance in the exhaust from said cell, dehydrating the exhaust from said cell, and utilizing the resultant dehydrated exhaust free of said trace substance to mix with a new sample to be analyzed in said cell.
 7. Apparatus for detecting the presence of a substance in a gaseous sample in the presence of substantial quantities of water vapor, which comprises an envelope having a gas inlet to which said sample is fed, means for forming ions from said sample in said envelope by ion-molecule reactions, means for applying a drift field to the ions in said envelope, means for producing an electrical output from said envelope in accordance with the velocity of predetermined ions in said drift field, means for mixing said gaseous sample with dry gas substantially free of said substance before said sample is fed to said inlet and until the partial pressure of said water vapor is of the order of 2 Torr or less, and means for maintaining substantially atmospheric Pressure in said envelope.
 8. Apparatus in accordance with claim 7, further comprising means for heating said envelope and said mixing means.
 9. Apparatus in accordance with claim 8, wherein said envelope and said mixing means are surrounded by thermal insulation.
 10. Apparatus in accordance with claim 7, wherein said mixing means comprises a venturi.
 11. Apparatus for detecting the presence of a substance in a gaseous sample, which comprises an envelope having a gas inlet to which said sample is fed, means for forming ions from said sample in said envelope, means for applying a drift field to the ions in said envelope, means for producing an electrical output from said envelope in accordance with the velocity of predetermined ions in said drift field, and means for mixing said gaseous sample with dry gas substantially free of said substance before said sample is fed into said inlet, said envelope having a gas outlet connected to said mixing means in a recirculation loop containing means for removing said substance.
 12. Apparatus in accordance with claim 11, wherein the last-mentioned means includes a molecular sieve.
 13. Apparatus in accordance with claim 11, wherein said recirculation loop contains dehydrating means.
 14. Apparatus in accordance with claim 11, wherein said recirculation loop includes means for forcing gas therethrough.
 15. Apparatus in accordance with claim 11, wherein said recirculation loop has means for venting excess gas to the atmosphere. 